My 1999 Mitsubishi Montero is acting strange:
draining the brand new battery in two days even when just parking in the garage.
Remote key does not open the locks, only clicking noise generated. Engine starts and runs while battery is still charged.
Is it possible that this condition is coming from faulty alternator?
Alternator is possible, but it is probably an accessory inside the vehicle.
The first thing I would check is the alternator charging voltage. With the vehicle running, measure the voltage at the battery with a voltmeter. It should be between 13.5 and 14.5 Volts. If it's below that, I would suspect the alternator.
I would also fully charge the battery and have a garage perform a load test. It's not unheard of to get a bad new battery.
Once those two checks are done it's time to find the drain...
The best way to check for drain is to use an ammeter, hooked inline with the negative cable at the battery. With everything off, and no key in the ignition, start pulling fuses until you find the circuit causing the drain. Your radio and ecu will probably cause a slight drain, and 50mA is not uncommon so don't think you have to get the drain down to 0. You are looking for a circuit causing over 500mA to drain your battery that fast.
Once you find the circuit causing the drain, you need to determine what component in that circuit is causing the problem. Damaged light sockets, bad solenoids and motors have been known to cause issues. It's always an adventure finding electrical problems, and don't forget a bad wire can also leak to ground and cause a drain.